1. Technical Field
The subject invention relates to a two-stroke crankcase scavenged internal combustion engine in which a piston ported scavenging air passage is arranged between a scavenging air inlet and the upper part of one or more transfer ducts. Fresh air is added proximate a top end of the transfer ducts and is intended to serve as a buffer against the air/fuel mixture below. Mainly, this buffer is lost out through the exhaust outlet during the scavenging process. In this way, both fuel consumption and exhaust emissions are reduced; less unburned fuel is released to the atmosphere which is wasted and is a pollutant. In a preferred application, the engine is intended to be incorporated into a handheld working tool.
2. Background of the Invention
Internal combustion engines of the two-stroke crankcase scavenged type are known. This configuration reduces fuel consumption and exhaust emissions, but in known designs, it is difficult to control the air/fuel ratio in these type of engines.
U.S. Pat. No. 5,425,346 discloses an engine with a somewhat different design than that which is described above. In this patent, channels are arranged in the piston of the engine, which at specific piston positions are aligned with ducts in the cylinder. Fresh air and/or exhaust gases can be added to the upper part of the transfer ducts. This only happens at the specific piston positions where the channels in the piston and the ducts in the cylinder are aligned. This happens both when the piston moves downwards and when the piston moves upwards far away from the top dead center position. To avoid unwanted flow in the wrong direction in the latter case, check valves are arranged at the inlet to the upper part of the transfer ducts. Inclusion of this type of check valve, which is often of the reed valve type, has a number of disadvantages. For instance, these check valves frequently have a tendency to come into resonant oscillations and can have difficulties coping with the high rotational speeds or cycles at which many two-stroke engines can operate. Besides, such a valve""s inclusion results in added cost and an increased number of engine components. The amount of fresh air added is varied through the use of a variable inlet; for example, an inlet that can be advanced or retarded in the work cycle. This is, however, a very complicated solution.
International Patent Application WO 98/57053 shows several different embodiments of an engine in which air is supplied to the transfer ducts via L-shaped or T-shaped channels in the piston. In this way, check valves are avoided. In all embodiments, the piston channel has, where it meets the respective transfer duct, a very limited height, which is essentially equal to the height of the actual transfer port. A consequence of this design is that the passage for the air delivery through the piston to the transfer port is opened significantly later than is the passage for the air/fuel mixture to the crankcase. The period for the air supply is consequently significantly shorter than the period for the supply of air/fuel mixture, where the period can be counted as crank angle or time. This can complicate the control of the total air-fuel ratio of the engine. This also results in that the amount of air that can be delivered to the transfer duct is significantly limited since the underpressure driving this additional or scavenging air has decreased substantially because the engine air inlet port has already been open during a certain period of time when the scavenging air supply is also opened. This implies that both the period and the driving force for the scavenging air supply are small in this configuration. Furthermore, the resistance to air flow in the L-shaped and the T-shaped ducts, as shown, becomes relatively high. This resistance is at least partly due to the cross section of the duct being small close to the transfer port and partly because of the sharp bend created by the L-shape or T-shape. In all, this contributes to increasing the flow resistance and to reducing the amount of air that can be delivered to the transfer ducts. This, in turn, reduces the possibilities to reduce fuel consumption and exhaust emissions by the arrangement.
The objects of the present invention(s) are achieved for a two-stroke combustion engine in accordance with the descriptions contained herein. The invention may take the form of a two-stroke combustion engine configured to include one or more piston ported air passages, each arranged from a scavenging air inlet that is equipped with a restriction valve. The restriction valve(s) are controlled by at least one engine parameter, such as the carburetor throttle control. The scavenging air inlet is connected to at least one connecting duct, each of which is channeled to a connecting port in the cylinder wall of the engine. The arrangement is configured so that each connecting port is connected with a flow path embodied in the piston when the piston is proximate the top dead center position. Each flow path in the piston extends to an upper part of a transfer duct fluidly connecting the engine""s cylinder to the crankcase. In one embodiment, the flow paths each take the form of a recess in a peripheral surface of the piston. Each recess is configured to, at certain times, commonly overlay a paired connecting and scavenging port. The recess moves through registration with a paired set of ports permitting scavenging air to be supplied toward the crankcase. Based on the arrangement thus disclosed, the period during which the engine air is provided to establish the air/fuel mixture and the period during which scavenging air is delivered to the engine in each cycle can be manipulated by varying one or more of the described features.
Regarding two-stroke internal combustion engines that are employed for powering hand-held machines, there are two general performance categories. A first of the two categories is typified by professional debranching saws in which quick acceleration to high operating speeds is desired. It is also desired that the greatest operating torques and power be produced at these higher running speeds, as opposed to the lower speeds that these type of engines pass through on the way up to high-speed operation. For reference purposes henceforth, these types of engines will be referred to as high speed/high torque engines. The second category of two-stroke engines is configured to produce maximum torque at lower speed. Tools that regularly employ such engines are typified by cutting saws such as those used to cut concrete. Operational speeds of these engines is desirably kept low, while at the same time delivering maximum torque and power in these low speed ranges. Additionally, the power curve for these types of engines can be characterized as having increasing torque ratios for decreasing engine speeds within relevant operational ranges. For reference purposes henceforth, these types of engines will be referred to as low speed/high torque engines.
The manipulation to two engine parameters has been found particularly useful in the design and control of these two-stroke internal combustion engines. With respect to the engines"" design, the length of the channel for the fuel/air mixture can be adjusted with respect to the length of the channel for the scavenging air. With respect to the engines"" control or operation, the relative time period for the supply of the fuel/air mixture versus the time period for the supply of scavenging air can be advantageously manipulated. In both instances, that is for the design and control engine parameters, a preferred range of values has been identified that encompasses both the high speed/high torque engines, as well as the low speed/high torque engines. Within these broader ranges, however, particularly preferred sub-ranges have been identified for the two engine groups.
In this regard, it has been found advantageous to regulate the relative periods of scavenging air supply time to engine air supply time for the air/fuel mixture to between about 0.7 and about 1.2. A particularly advantageous ratio has been discovered to be between about 0.7 and about 1.0 for high speed/high torque engines and between about 0.9 and about 1.2 for low speed/high torque engines. This variable can be manipulated to produce desired characteristics under different operating conditions; for example, one ratio may be induced for potentiated performance during maximum torque production when high amounts of air are desired to be taken into the crankcase for having the overall effect of leaning the fuel/air mixture supplied to the engine""s cylinder. For simplicity, these relative periods can be measured based on angular travel of the crank and/or time.
Regarding the relative channel lengths, it has been found advantageous to configure the channels so that the relative length of the channel for the fuel/air mixture to the length of the channel for the scavenging air is between about 0.3 and about 1.4. A particularly advantageous ratio has been discovered to be between about 0.4 and about 0.5 for high speed/high torque engines and between about 0.3 and about 0.6 for low speed/high torque engines. As with manipulation of the relative supply time periods addressed immediately above, this variable can also be manipulated to produce desired characteristics under different operating conditions.
With respect to the relative lengths of the two flow channels or passages, the passage through which scavenging air travels is generally measured between the scavenging air inlet, usually controlled by a restrictive valve, and a terminal inlet port at the crankcase. Along this path, the scavenging air traverses the connecting duct, the flow path at the piston, and the transfer duct. The passage through which the engine air, and then the engine air/fuel mixture travels is generally measured from the engine air inlet, passed the station where fuel is added, and on to a port proximate the engine""s crankcase.
In an exemplary embodiment, the length of the engine air passage into which fuel is added, Li, is greater than 0.6 times the total length of the piston ported air passage Lai and the length of the transfer duct Ls, i.e. 0.6xc3x97(Lai+Ls) but smaller than 1.4 times the same length, i.e. 1.4xc3x97(Lai+Ls).
By adapting the length of the ducts leading the air to the crankcase in relation to the length of the inlet duct, the control of the engine can be simplified. By adapting these two duct systems in relation to each other, the flow in each system will vary concurrently with the flow in the other system. In this manner a carburetor in the inlet system could supply the correct amount of fuel to the engine irrespective of load variations and other factors impacting the engine""s operation. In one respect, high speed engines having relatively short, low volume, scavenging channels can be dimensioned so that they do not hold all the scavenging air that is delivered to the engine during maximum torque speed because of their being too small, but that can hold all of the scavenging air, which is a lesser amount, delivered at maximum power speed. Manipulation of these relative lengths is an aspect of the presently disclosed invention used to adjust the fuel/air ratio curve of an engine. Because the total fuel/air ratio is usually at its richest around maximum torque demand conditions, manipulation of the relative lengths of the channels is taught to be manipulated for desirably leaning the overall mixture, including mixing of the fuel/air supply from the carburetor with scavenging air amounts at the crankcase.
Because at least one connecting port in the engine""s cylinder wall is arranged so that it in connection with piston positions at the top dead center is connected with flow paths embodied in the piston, the supply of fresh air to the upper part of the transfer ducts can be arranged entirely without check valves. This can take place because at piston positions at or near the top dead center there is an underpressure in the transfer duct in relation to the ambient air. Thus a piston ported air passage without check valves can be arranged, which is a substantial advantage. Because the air supply has a relatively long period, a large amount of air can be delivered so that a high exhaust emissions reduction effect can be achieved. Control is applied by means of a restriction valve in the air inlet, controlled by at least one engine parameter. Such control is of a significantly less complicated design than a variable inlet. The air inlet has preferably two connecting ports, which in one embodiment are so located that the piston is covering them at its bottom dead center. The restriction valve can suitably be controlled by the engine speed, alone or in combination with another engine parameter. These and other characteristics and advantages are clarified in the detailed description of the different embodiments, supported by the included drawing figures.